The accurate reproduction of patterns on the surface of a semiconductor substrate is critical to the proper fabrication of semiconductor devices. The semiconductor substrate may have undergone previous fabrication processes and may already feature layers and structures created by those fabrication processes. Improperly reproduced patterns can result in semiconductor devices that do not operate to design specifications or that do not operate at all. For example, transistors can be created with improperly sized gates; conductors can be created that are short circuited or open circuited with other conductors or devices; structures can be created with wrong geometries, and so forth. Improperly reproduced patterns can reduce the yield of the fabrication process, thereby increasing the overall cost of the product. The reproduction process typically involves the use of optical lithography to reproduce the patterns onto the surface of the semiconductor substrate followed by a variety of processes either to subtract (for example, etch) or to add (for example, deposit) materials from and to the semiconductor substrate.
However, as the dimensions of the structures making up the patterns continue to become smaller, their sizes approach the wavelengths of the light used in optical lithography. Interference and processing effects can cause distortion and deviation in the mask's patterns as they are reproduced onto the semiconductor substrate.
Shrinking device geometries also have a particularly acute affect on patterning small contact holes. As contact holes become smaller, masks used to pattern contact holes require smaller apertures. The smaller the aperture, the more difficult it is to get enough light through the aperture to adequately expose the resist disposed on the semiconductor wafer below. If not enough light is used to expose the mask, a pattern will not print on the resist below. If, on the other hand, the mask and semiconductor wafer is exposed with a high intensity of light, the exposed pattern below becomes washed out and sidelobes are exposed beyond the desired perimeter of the exposed area. Consequently, the generation and exposure of small contact whole regions require a very narrow lithographic process window, thereby limiting the range of exposure settings that will produce an adequate exposure. These exposure settings can include illumination or dose, focus, numerical aperture, and light coherence factor, sigma.
In the field of small, densely packed application is using small lithographic geometries, what is needed is a method that can optimally expose a small geometric feature, such as a contact hole.